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Ocean Drilling Program Initial Reports Volume Sager, W.W., Winterer, E.L., Firth, J.V., et al., 1993 Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 143 9. SITE 869l Shipboard Scientific Party2 HOLE 869A Principal results: Site 869 (proposed Site Syl-3) is situated 45 nmi (83 km) southwest of the atoll-guyot pair, Pikinni Atoll (formerly Bikini) and Wode- Date occupied: 3 May 1992 jebato Guyot (formerly Sylvania). Drilling at this location was planned to Date departed: 4 May 1992 provide a basinal reference section for comparison to Leg 144 drill holes on the summit of Wodejebato and prior drilling on Pikinni. Site-survey data Time on hole: 1 day 17 hr suggested that a thick, layered succession of sediments exists at the site that Position: ll°0.091'N, 164°44.969'E consists mainly of turbidites. Volcanic basement is not obvious in the Bottom felt (rig floor, m; drill-pipe measurement): 4837.8 site-survey seismic lines, so drilling was not expected to encounter basement basalt. Approximately 850 m of penetration was planned, and the expecta- Distance between rig floor and sea level (m): 11.1 tion was to bottom in Cretaceous volcaniclastics produced by constructional Water depth (drill-pipe measurement from sea level, m): 4826.7 volcanism on either or both Wodejebato and Pikinni. Total depth (rig floor, m): 5004.3 The location of Site 869, at 1 l°00.09'N, 164°45.02'E, was chosen on the seafloor adjacent to both Pikinni and Wodejebato, close enough to Penetration (m): 166.5 receive sediments shed mainly by these edifices, but far enough away so Number of cores (including cores with no recovery): 18 that both would contribute. It was assumed that sedimentation at an adjacent basinal hole would parallel and complement that on the atoll- Total length of cored section (m): 166.5 guyot pair. For example, erosional hiatuses on the summit might be Total core recovered (m): 129.27 expected to produce sediments on the archipelagic apron, thus filling Core recovery (%): 77 "missing chapters" in the geologic history provided by summit holes. Drilling at Site 869 was undertaken to address the following primary goals: Oldest sediment cored: Depth (mbsf): 166.5 1. Determining the history of volcanism on Pikinni/Wodejebato; Nature: Chert 2. Developing a model of Cretaceous, mid-ocean guyot/atoll carbonate Age: Eocene platform formation and evolution for the Marshall Islands; Measured velocity(km/s): 1.4—1.8 3. Comparison of the timing and effects of fluctuations in relative sea level among Pacific guyots and relation to worldwide sea-level curves; and HOLE 869B 4. Deciphering the cause(s) for the drowning(s) of some Marshall Islands guyots (such as Wodejebato) vs. the existence of Cenozoic reefs Date occupied: 4 May 1992 on nearby atolls (such as Pikinni). Date departed: 15 May 1992 Time on hole: 10 days 20 hr Two holes were drilled at Site 869, an APC/XCB hole (Hole 869A), and an RCB hole (Hole 869B), the latter located about 93 m north of the Position: ll°0.093'N, 164°45.019'E former. The operational plan was to use the APC/XCB combination to Bottom felt (rig floor, m; drill-pipe measurement): 4837.8 obtain relatively undisturbed cores from the upper 300 to 400 m of the sedimentary column. In analogy to Site 462, hard chert layers were Distance between rig floor and sea level (m): 11.1 expected to frustrate drilling at about 300 to 400 mbsf, and a round-trip Water depth (drill-pipe measurement from sea level, m): 4826.7 was planned to change to an RCB bit and begin a second hole. Chert and Total depth (rig floor, m): 5634.0 porcellanite were encountered at much shallower depths than expected, and Hole 869A was terminated at a depth of only 166.5 mbsf. Hole 869B Penetration (m): 796.2 was washed to 140.0 mbsf and cored continuously to a total depth of Number of cores (including cores with no recovery): 69 796.2 mbsf. Total length of cored section (m): 796.2 Core recovery in Hole 869A, drilled with the APC/XCB combination, was mostly excellent, with the average being over 100% for the APC cores Total core recovered (m): 252.04 and 77.6% overall. In Hole 869B, recovery was variable (see "Operations" Core recovery (%): 31 section, this chapter). In cherty sections, recovery was generally low, and Oldest sediment cored: we obtained only a few pebbles of chert in some cores. In contrast, recovery Depth (mbsf): 796.2 in Cretaceous volcaniclastic turbidites was typically in excess of 60%. In general, recovery increased downhole in Hole 869B, averaging about 20% Nature: Volcanic sandstone from 200 to 500 mbsf and increasing to 70% at the bottom of the hole. Age: Cenomanian Measured velocity (km/s): 3.0^.0 Using combined microfossil and macrofossil biostratigraphy, visual core descriptions augmented with smear-slide and thin-section data, physical properties data, and downhole logs, three principal stratigraphic units were recognized (see "Lithostratigraphy" section, this chapter). In ' Sager, W.W., Winterer, E.L., Firth, J.V., et al., 1993. Proc. ODP, Init. Repts., 143: College Station, TX (Ocean Drilling Program). stratigraphic order, from bottom to top, the lithologic divisions are de- " Shipboard Scientific Party is as given in list of participants preceding the contents. scribed as follows: 297 SITE 869 1. Unit III (796.2-207.7 mbsf> Middle-upper Cenomanian to upper panian section, from approximately 400 to 225 mbsf. Poor recovery Campanian-lower Maastrichtian volcaniclastics interlayered with nanno- prevents the dating of the interval from 225 to 207 mbsf, but this interval fossil and radiolarian claystone. This unit is characterized by numerous gray resembles the Cretaceous volcaniclastic section in downhole logs. Into the to green volcaniclastic sandstones and breccias intercalated with lighter- Cenozoic, lithology changes to cherts and radiolarian-nannofossil oozes. colored claystones. Seven lithologic subunits can be recognized on the basis The uppermost cored interval of Hole 869B is early Paleocene to early of changes in the mix, grain sizes of, or depositional style of the dominant Eocene in age. Hole 869A yielded entirely Cenozoic sediments that range components. Subunits IIIF (780.7-653.3 mbsf), HIE (653.3-536.1 mbsf), in age from middle Eocene to early Miocene. The stratigraphic progression and IIIC (487.8^58.8 mbsf), and III A (381.5-207.7 mbsf) consist mostly at Site 869 is punctuated by five recognized hiatuses. The missing strata of volcaniclastic sandstone interbedded with claystone. These similar sub- are (1) the upper Coniacian to lower Santonian; (2) upper Maastrichtian to units are punctuated by Subunits IIIG (796.2-780.7 mbsf), a volcanic lower Paleocene; (3) part of the upper Paleocene; (4) a section including siltstone with calcareous claystone, Subunits HID (536.1^87.8 mbsf), a the Oligocene/Miocene boundary; and (5) post-early Miocene age sedi- volcanic breccia, and IIIB (458.8-381.5 mbsf), which consists mainly of ments missing at the seafloor. Of these, the most prominent is the upper radiolarian-rich claystone. Maastrichtian-lower Paleocene hiatus, which spans up to 10 m.y. and The volcaniclastic layers consist mainly of sand-size grains, deposited includes the Cretaceous/Tertiary boundary. as turbidites. Basaltic clasts are common, in places forming breccias, but Paleomagnetic measurements of the sediments cored in Hole 869A were also occurring within a fine-grained matrix and implying transportation by ruined by pervasive rust contamination of the APC/XCB cores. Measure- grain flow. Most clasts are subangular to subrounded, suggesting a moderate ments of discrete samples and archive-half samples from the Cretaceous transport distance. The largest clasts are up to 80 mm in diameter, and most section of Hole 869B showed normal polarity down to Core 143-869B-21R, have been affected by light to moderate alteration. The volcaniclastic layers reversed polarity from Cores 143-869B-21R to -26R, and normal polarity contain clinopyroxene, palagonite, feldspar, zeolite, epidote, and chlorite below. This reversed zone has been interpreted as Chron 33R, the first grains, attesting to a basaltic parentage. Zeolites are a common cement. reversed polarity epoch after the Cretaceous Quiet Period (Cretaceous Long Claystones occur abundantly in some parts of the section (e.g., Sub- Normal Superchron, K-N). Paleolatitudes from Hole 869B samples are in unit IIIB), but rarely in others (e.g., Subunit IIIF). They are locally calcare- low southern latitudes, 10°-20°S. ous, siliceous, and/or zeolitic. Radiolarian and nannofossil concentrations Because shipboard magnetic stratigraphy interpretations were ham- are variable; in some layers, these fossils are dispersed through the matrix pered by rust contamination in Hole 869A and by the few reversals in the and in others they are concentrated in millimeter-scale beds. Within the Cretaceous sections drilled in Hole 869B, calculated sedimentation rates Cenomanian section, unequivocal shallow-water debris are absent. Shal- were based solely on biostratigraphy. Sediments accumulated in the Ceno- low-water biogenic fragments occur only rarely in the Turanian to Maas- manian section, from the bottom of the hole up to about 470 mbsf, at the trichtian beds above, mainly in Subunits IIIA, IIIB, and IIIC. Bivalve shell, rapid rate of 60 m/m.y. or greater. In the Cretaceous section above, up to a gastropod, echinoid, red-algal, and recrystallized skeletal fragments were depth of about 207 mbsf, the overall rate was about 15 m/m.y. A hiatus of found as were orbitoid foraminifers, micritic ooids, peloids, and glau- 10 m.y. duration may separate the Cenozoic and Mesozoic parts of the conite. Coalified woody fragments were found in Core 143-869B-51R, at sedimentation curve.
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